Electrolytic manganese dioxide for lithium primary battery, and lithium primary battery using the same

ABSTRACT

Electrolytic manganese dioxide for lithium primary batteries has a sodium content of 0.05 to 0.2% by mass, and a pH of 5 to 7 as measured according to JIS-K-1467. Using this electrolytic manganese dioxide as a positive electrode active material for lithium primary batteries enables the batteries to be excellent in both initial discharge characteristics and long-term discharge characteristics.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/681,500, filed on Apr. 2, 2010, which is the U.S. National Phaseunder 35 U.S.C. §371 of International Application No. PCT/JP2009/003122,filed on Jul. 6, 2009, which in turn claims the benefit of JapaneseApplication No. 2008-178819, filed on Jul. 9, 2008, the disclosures ofwhich Applications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to electrolytic manganese dioxide forlithium primary batteries, manufacturing method therefor, and a lithiumprimary battery using the same as a positive electrode active material.

BACKGROUND ART

Lithium primary batteries use lithium and other light metals as anegative electrode active material, and manganese dioxide or graphitefluoride as a positive electrode active material. These batteries haveunique features including high voltage, high energy density, lowself-discharge, and an extremely long storage life, and hence are usedin various electronic devices.

Among the materials used as a positive electrode active material,manganese dioxide is very popular because it is inexpensive andabundant, and as a positive electrode active material for lithiumprimary batteries, electrolytic manganese dioxide is commonly used dueto its excellent discharge performance and long-term storageperformance.

Electrolytic manganese dioxide is generally electrolytically synthesizedin a sulphuric acid solution containing manganese ions, and therefore,is required to be neutralized with alkali when used as a positiveelectrode active material for lithium primary batteries. Popularexamples of the alkali used for neutralization are ammonia and sodiumhydroxide.

Electrolytic manganese dioxide prepared by neutralization with ammonia(hereinafter, ammonia-neutralized product) is widely used for lithiumprimary batteries. The ammonia-neutralized product, however, ismanufactured by only a few manufacturers, and therefore, is lessavailable and more expensive than electrolytic manganese dioxideprepared by neutralization with sodium hydroxide (hereinafter,sodium-neutralized product). As another disadvantage, when used as apositive electrode active material for lithium primary batteries, theammonia-neutralized product requires a dedicated exhaust system toensure working conditions because it causes ammonia to volatize, givingoff a pungent smell when heat-treated to remove moisture.

The sodium-neutralized product, on the other hand, is mainly used as apositive electrode active material for dry batteries. Thesodium-neutralized product generally contains 0.3 to 0.5% by mass ofsodium, which may reduce the discharge performance when used for lithiumprimary batteries. The reason for the reduction is that the sodium inthe sodium-neutralized product is deposited on the lithium used as anegative electrode active material and forms a resistance film thereon.The deposition is more significant as the battery is stored at a highertemperature and for a longer period. This is why the sodium-neutralizedproduct is little used as a positive electrode active material forlithium primary batteries although it is easily available.

The sodium-neutralized product, however, is inexpensive andmass-produced, and therefore, it is a valuable attempt from anindustrial viewpoint to make full use of this product as electrolyticmanganese dioxide for lithium primary batteries.

To achieve this attempt, it has been suggested that electrolyticmanganese dioxide is sodium-neutralized in such a manner that theneutralized electrolytic manganese dioxide has a minimum sodium contentin the range of 0.05 to 0.2% by mass (Patent Literature 1, for example).

The sodium-neutralized product having a minimum sodium content, however,contains a large amount of sulfuric acid residues, making its pH as lowas 2 to 4. When sintered and used as a positive electrode activematerial for lithium primary batteries, such a sodium-neutralizedproduct with a low pH increases the battery internal resistance when aweak discharge is continued for a long time such as over one year,although its initial discharge performance is excellent.

Patent Literature 1: Japanese Patent Unexamined Publication No.2001-236957

SUMMARY OF THE INVENTION

The present invention relates to electrolytic manganese dioxide forlithium primary batteries prepared from a sodium-neutralized product.The invention has an object of providing electrolytic manganese dioxidethat offers both excellent initial discharge characteristics andexcellent long-term discharge performance when used for a lithiumprimary battery, a method for producing the electrolytic manganesedioxide, and a lithium primary battery using the electrolytic manganesedioxide as a positive electrode active material, thereby being excellentin long-term discharge performance.

The electrolytic manganese dioxide for lithium primary batteriesaccording to the present invention has a sodium content of 0.05 to 0.2%by mass, and a pH of 5 to 7 as measured according to JIS-K-1467. Thelithium primary battery using the electrolytic manganese dioxideaccording to the present invention as a positive electrode activematerial can reduce an increase in the battery internal resistance evenwhen a weak discharge is continued for a long time such as over oneyear.

The method for producing the electrolytic manganese dioxide for lithiumprimary batteries according to the present invention includes preparingneutralized electrolytic manganese dioxide by neutralizing electrolyticmanganese dioxide with sodium hydroxide, the electrolytic manganesedioxide being electrosynthesized in an acid electrolytic bath; andwashing the neutralized electrolytic manganese dioxide with water insuch a manner that water-washed electrolytic manganese dioxide has asodium content of 0.05 to 0.2% by mass, and a pH of 5 to 7 as measuredaccording to JIS-K-1467.

The lithium primary battery according to the present invention uses, asa positive electrode active material, electrolytic manganese dioxidehaving a sodium content of 0.05 to 0.2% by mass, and a pH of 5 to 7 asmeasured according to JIS-K-1467. Using this electrolytic manganesedioxide can reduce the formation of a resistance film containing sodiumor manganese on the negative electrode, enabling the lithium primarybattery to be excellent in both initial discharge performance andlong-term discharge performance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic sectional view of a lithium primary batteryaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The following is a description of an embodiment of the presentinvention. Note that the following embodiment is one example of thepresent invention, and does not limit the technical scope of theinvention.

The sodium content of the electrolytic manganese dioxide for lithiumprimary batteries is measured by ICP analysis. In the case of producinga lithium primary battery using the electrolytic manganese dioxideprepared from a sodium-neutralized product as a positive electrodeactive material, sodium elutes from the electrolytic manganese dioxidewhen its sodium content is larger than 0.2% by mass. The eluted sodiumis deposited on the lithium used as the negative electrode activematerial and forms a resistance film thereon, reducing the batterydischarge performance. When the electrolytic manganese dioxide has a pHof less than 5 as measured according to JIS-K-1467, sulfuric acidcomponents remaining in the electrolytic manganese dioxide react withthe small amount of water in the battery and generate an acid, causingmanganese ions to elute from the electrolytic manganese dioxide during along discharge. The eluted manganese ions are deposited on the lithiumused as the negative electrode active material and form resistance filmthereon, causing an increase in battery internal resistance.

Note that it is impossible to produce electrolytic manganese dioxidehaving a pH greater than 7 and a sodium content of 0.05 to 0.2% by massby means of electrolyzing it in an acid electrolytic bath and thensodium-neutralizing it.

Also note that when the sodium content of the sodium-neutralized productis less than 0.05% by mass, it is difficult to have a pH of 5 or greaterby an after-mentioned washing treatment with water.

The following is a brief description of the pH measurement methodspecified in JIS-K-1467. First, 15 g of manganese dioxide as a sample isput in a 200 ml Erlenmeyer flask, and 75 ml of a 20% aqueous NH₄Clsolution is added thereto so as to prepare a sample solution. Theaqueous NH₄Cl solution contains 100 ml of water and 20 g of NH₄Cldissolved therein. Next, the sample solution is stirred with a magneticstirrer for 30 minutes. The stirring is performed at a rate that doesnot cause the solution to spatter in the flask. After the stirring, theflask is left for five to ten minutes at an inclination angle of 30degrees. Then, 50 ml of the supernatant solution is collected andmeasured for its pH by a digital pH meter or the like. The obtainedvalue is considered to be the pH of the manganese dioxide.

The following is a description of a method for measuring a sodiumcontent by ICP analysis. First, 1 g of manganese dioxide as a sample isput in a 200 ml beaker, and 20 ml of hydrochloric acid (a 50% by volumeaqueous solution) is added thereto. The resulting solution is heateduntil the manganese dioxide is dissolved, and then cooled. The cooledsolution is filtered, and pure water is added thereto so as to make 100ml of the solution. The resulting solution is subjected to standardaddition method using an atomic absorption spectrometer so as todetermine the quantity of sodium.

The following is a description of a method for producing theelectrolytic manganese dioxide for lithium primary batteries accordingto the embodiment.

First, electrolytic manganese dioxide is prepared which iselectrosynthesized in an acid electrolytic bath containing a sulphuricacid solution. Then, the electrolytic manganese dioxide is neutralizedwith aqueous sodium hydroxide so as to prepare neutralized electrolyticmanganese dioxide. The neutralization is performed using aqueous sodiumhydroxide that contains 2.0 to 10.0 g of sodium hydroxide per 1 kg ofelectrolytic manganese dioxide. As a result, the neutralizedelectrolytic manganese dioxide has a sodium content of 0.05 to 0.5% bymass. The neutralized electrolytic manganese dioxide thus obtainedgenerally has a pH of 2 to 4 as measured according to JIS-K-1467.

Next, the neutralized electrolytic manganese dioxide is stirred andwashed with water, centrifuged to remove water, and dried, thuspreparing water-washed electrolytic manganese dioxide. The washing isperformed in such a manner that the water-washed electrolytic manganesedioxide has a sodium content of 0.05 to 0.2% by mass, and a pH of 5 to 7as measured according to JIS-K-1467.

When a large amount of sodium hydroxide is used for the neutralization,and as a result, the sodium content of the neutralized electrolyticmanganese dioxide is 0.4 to 0.5% by mass, a large amount of water isrequired for washing to make the sodium content of the water-washedelectrolytic manganese dioxide 0.2% by mass or less. In the contrary,when a small amount of sodium hydroxide is used for the neutralization,and as a result, the sodium content of the neutralized electrolyticmanganese dioxide is 0.05% by mass or more and less than 0.1% by mass,the neutralized electrolytic manganese dioxide has a low pH. In thiscase, a large amount of water is required to wash out sulfuric acidcomponents and to make the water-washed electrolytic manganese dioxidehave a pH of 5 or greater. Therefore, to save water for the washing, thesodium content of the neutralized electrolytic manganese dioxide ispreferably 0.1 to 0.4% by mass.

In general, when manganese dioxide is used as a positive electrodeactive material for lithium primary batteries, it is necessary to removethe water in the crystal of the manganese dioxide by a heat treatment at300° C. or above. In the method for producing electrolytic manganesedioxide for lithium primary batteries according to the presentinvention, the heat treatment can be performed either before or afterwashing because it does not change the sodium content or the pH of theelectrolytic manganese dioxide or affect the battery characteristics.

A lithium primary battery shown in FIG. 1 is produced using as apositive electrode active material the electrolytic manganese dioxideprepared as described above. FIG. 1 is a schematic sectional view of thelithium primary battery according to the embodiment of the presentinvention.

This lithium primary battery includes positive electrode 1 containingthe above-described electrolytic manganese dioxide as an activematerial, and negative electrode 2 containing lithium as an activematerial. Positive electrode 1 and negative electrode 2 are spirallywound together with separator 3 therebetween so as to form an electrodeassembly. The electrode assembly is put into case 9 together with anon-aqueous electrolytic solution (not shown). Case 9 has an openingsealed with sealing plate 8, which is joined to lead 4 connected to thecore sheet of positive electrode 1. Case 9 is also joined to lead 5connected to negative electrode 2. The electrode assembly is providedwith upper insulating plate 6 and lower insulating plate 7 for internalshort circuit protection.

Positive electrode 1 is produced as follows. The electrolytic manganesedioxide prepared by neutralization and washing with water as describedabove is mixed with a conductive agent, added with a binder and water,and then kneaded together to prepare a positive-electrode mixture. Theconductive agent can be graphite powder such as artificial graphite ornatural graphite, or a mixture of graphite powder and carbon black suchas acetylene black. The amount of the conductive agent to be used can belarge enough to fully inject the electrolytic manganese dioxide and toform a conductive path so as to reduce the electric resistance ofpositive electrode 1. In particular, it is preferable that theconductive agent contains 4 to 8 parts by weight of graphite per 100parts by weight of the electrolytic manganese dioxide. Next, thepositive-electrode mixture is injected into a core sheet having a meshstructure or fine pores such as an expanded metal, a net, or aperforated metal, then rolled, and cut in size. Then, part of thepositive-electrode mixture is peeled off to which lead 4 is welded,thereby producing belt-shaped positive electrode 1.

Negative electrode 2, on the other hand, which is also belt-shaped, iscomposed of metallic lithium or a lithium alloy such as Li—Al, Li—Sn,Li—NiSi, or Li—Pb.

The solvent of the non-aqueous electrolytic solution is not particularlylimited as long as it is an organic solvent generally used in anon-aqueous electrolytic solution for lithium batteries. Morespecifically, it is possible to use γ-butyrolactone, propylenecarbonate, ethylene carbonate, or 1,2-dimethoxyethane either alone or incombination.

The non-aqueous electrolytic solution contains a supporting electrolyte,which can be lithium tetrafluoroborate, lithium phosphorus hexafluoride,lithium trifluoromethanesulfonate, or LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, orLiN(CF₃SO₂)(C₄F₉SO₂) having an imide bond in the molecular structure.

Separator 3 can be made of a woven or nonwoven polyolefin cloth, amicroporous film, or the like.

The effect of the embodiment will be described in specific examplesbelow. The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 3.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.10%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then, theheat-treated electrolytic manganese dioxide is stirred and washed withwater which is added in a ratio of 10 kg per 1 kg of the heat-treatedelectrolytic manganese dioxide. After the washing with water, theelectrolytic manganese dioxide is centrifuged to remove water, anddried, thus preparing water-washed electrolytic manganese dioxide havinga sodium content of 0.05% by mass, and a pH of 5.0 as measured accordingto JIS-K-1467.

To be used as a positive electrode active material, the water-washedelectrolytic manganese dioxide is added with 5% by mass of graphite as aconductive agent, and 2% by mass of polytetrafluoroethylene as a binder.Then, the resulting mixture is kneaded together with 35% by mass of purewater so as to prepare a wet positive-electrode mixture. The wetpositive-electrode mixture and a 0.1 mm thick stainless expanded metalare fed together between two rotating rollers rotating at the same speedso as to inject the positive-electrode mixture into the expanded metal,thereby producing a mixture sheet. The mixture sheet is dried, rolled bya roller press, and cut in size (0.40 mm in thickness, 26 mm in width,and 235 mm in length), thereby preparing positive electrode 1.

Negative electrode 2 is a lithium metal plate, which is cut in size(0.18 mm in thickness, 24 mm in width, and 260 mm in length). Positiveelectrode 1 and negative electrode 2 thus prepared are spirally woundtogether with the separator made of a microporous polyethylene film,thus forming the electrode assembly. The electrode assembly is put intocase 9. Next, stainless lead 4 connected to the core sheet of positiveelectrode 1 is connected to the positive terminal of sealing plate 8,and nickel lead 5 connected to negative electrode 2 is connected to case9. Then, the unillustrated non-aqueous electrolytic solution is injectedinto case 9, and the opening of case 9 is sealed. As a result, acylindrical manganese dioxide lithium primary battery shown in FIG. 1 isobtained which has a diameter of 17 mm and a height of 33.5 mm. Ten suchbatteries are produced. The non-aqueous electrolytic solution isprepared by dissolving lithium trifluoromethanesulfonate as a supportingelectrolyte at a concentration of 0.5 mol/L in a non-aqueous solvent.The non-aqueous solvent is a mixed solvent of propylene carbonate anddimethoxyethane in a volume ratio of 1:1. The manganese dioxide lithiumprimary batteries thus produced are referred to as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 7.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.30%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then, theheat-treated electrolytic manganese dioxide is stirred and washed withwater which is added in a ratio of 10 kg per 1 kg of the heat-treatedelectrolytic manganese dioxide. After the washing with water, theelectrolytic manganese dioxide is centrifuged to remove water, anddried, thus preparing water-washed electrolytic manganese dioxide havinga sodium content of 0.20% by mass, and a pH of 5.0 as measured accordingto JIS-K-1467. Batteries “B” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”. The electrolytic manganese dioxide, which hasbeen prepared by electrolyzation in a sulfuric acid bath is neutralizedby adjusting the concentration in the aqueous sodium hydroxide tocontain 9.0 g of sodium hydroxide per 1 kg of the electrolytic manganesedioxide. The neutralized electrolytic manganese dioxide has a sodiumcontent of 0.40% by mass. The neutralized electrolytic manganese dioxideis filtered, dried, and heat-treated at 400° C. for four hours. Then,the heat-treated electrolytic manganese dioxide is stirred and washedwith water which is added in a ratio of 20 kg per 1 kg of theheat-treated electrolytic manganese dioxide. After the washing withwater, the electrolytic manganese dioxide is centrifuged to removewater, and dried, thus preparing water-washed electrolytic manganesedioxide having a sodium content of 0.20% by mass, and a pH of 7.0 asmeasured according to JIS-K-1467. Batteries “C” are produced using thewater-washed electrolytic manganese dioxide thus obtained, but otherwisein the same manner as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 4.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.15%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then, theheat-treated electrolytic manganese dioxide is stirred and washed withwater which is added in a ratio of 20 kg per 1 kg of the heat-treatedelectrolytic manganese dioxide. After the washing with water, theelectrolytic manganese dioxide is centrifuged to remove water, anddried, thus preparing water-washed electrolytic manganese dioxide havinga sodium content of 0.05% by mass, and a pH of 7.0 as measured accordingto JIS-K-1467. Batteries “D” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 5.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.20%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then, theheat-treated electrolytic manganese dioxide is stirred and washed withwater which is added in a ratio of 10 kg per 1 kg of the heat-treatedelectrolytic manganese dioxide. After the washing with water, theelectrolytic manganese dioxide is centrifuged to remove water, anddried, thus preparing water-washed electrolytic manganese dioxide havinga sodium content of 0.10% by mass, and a pH of 6.0 as measured accordingto JIS-K-1467. Batteries “E” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 9.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.40%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then the heat-treatedelectrolytic manganese dioxide is stirred and washed with water which isadded in a ratio of 10 kg per 1 kg of the heat-treated electrolyticmanganese dioxide. After the washing with water, the electrolyticmanganese dioxide is centrifuged to remove water, and dried, thuspreparing water-washed electrolytic manganese dioxide having a sodiumcontent of 0.25% by mass, and a pH of 6.0 as measured according toJIS-K-1467. Batteries “F” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 5.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.15%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then, theheat-treated electrolytic manganese dioxide is stirred and washed withwater which is added in a ratio of 5 kg per 1 kg of the heat-treatedelectrolytic manganese dioxide. After the washing with water, theelectrolytic manganese dioxide is centrifuged to remove water, anddried, thus preparing water-washed electrolytic manganese dioxide havinga sodium content of 0.10% by mass, and a pH of 4.5 as measured accordingto JIS-K-1467. Batteries “G” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 5.0 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.20%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. The heat-treatedelectrolytic manganese dioxide has a pH of 4.5 as measured according toJIS-K-1467. Batteries “H” are produced using the heat-treatedelectrolytic manganese dioxide without being washed with water, butotherwise in the same manner as batteries “A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 1.5 g of sodiumhydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.05%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then, theheat-treated electrolytic manganese dioxide is stirred and washed withwater which is added in a ratio of 20 kg per 1 kg of the heat-treatedelectrolytic manganese dioxide. After the washing with water, theelectrolytic manganese dioxide is centrifuged to remove water, anddried, thus preparing water-washed electrolytic manganese dioxide havinga sodium content of 0.03% by mass, and a pH of 4.5 as measured accordingto JIS-K-1467. Batteries “I” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”.

The electrolytic manganese dioxide that has not been neutralized withaqueous sodium hydroxide has a sodium content of 0.01% by mass. Thiselectrolytic manganese dioxide is heat-treated at 400° C. for fourhours. The heat-treated electrolytic manganese dioxide is stirred andwashed with water which is added in a ratio of 10 kg per 1 kg of theheat-treated electrolytic manganese dioxide. After the washing withwater, the electrolytic manganese dioxide is centrifuged to removewater, and dried, thus preparing water-washed electrolytic manganesedioxide. This water-washed electrolytic manganese dioxide has a sodiumcontent of 0.01% by mass, which is the same as before being washed withwater, and has a pH of 2.0 as measured according to JIS-K-1467.Batteries “J” are produced using the water-washed electrolytic manganesedioxide thus obtained, but otherwise in the same manner as batteries“A”.

The electrolytic manganese dioxide, which has been prepared byelectrolyzation in a sulfuric acid bath is neutralized by adjusting theconcentration in the aqueous sodium hydroxide to contain 10.0 g ofsodium hydroxide per 1 kg of the electrolytic manganese dioxide. Theneutralized electrolytic manganese dioxide has a sodium content of 0.50%by mass. The neutralized electrolytic manganese dioxide is filtered,dried, and heat-treated at 400° C. for four hours. Then the heat-treatedelectrolytic manganese dioxide is stirred and washed with water which isadded in a ratio of 40 kg per 1 kg of the heat-treated electrolyticmanganese dioxide. After the washing with water, the electrolyticmanganese dioxide is centrifuged to remove water, and dried, thuspreparing water-washed electrolytic manganese dioxide having a sodiumcontent of 0.20% by mass, and a pH of 7.0 as measured according toJIS-K-1467. Batteries “K” are produced using the water-washedelectrolytic manganese dioxide thus obtained, but otherwise in the samemanner as batteries “A”.

Five of each of the batteries “A” to “K” thus produced are subjected todischarge at 500 mA at room temperature. The other five of each of thebatteries “A” to “K” are subjected to constant-resistance discharge at300 kΩ at room temperature and measured for internal resistance one yearlater. Table 1 below shows the sodium contents of the electrolyticmanganese dioxides before and after being washed with water, the pH ofthe electrolytic manganese dioxides after being washed with water. Notethat each value indicates a mean value of five batteries.

TABLE 1 internal resistance one sodium content discharge year later when(% by mass) at 500 discharged at before after mA^((note 1)) 300 kΩ^((note 2)) washing washing pH (initial) (Ω) batteries 0.10 0.05 5.0 1000.2 A batteries 0.30 0.20 5.0 98 0.2 B batteries 0.40 0.20 7.0 98 0.2 Cbatteries 0.15 0.05 7.0 100 0.2 D batteries 0.20 0.10 6.0 100 0.2 Ebatteries 0.40 0.25 6.0 94 0.2 F batteries 0.15 0.10 4.5 100 0.6 Gbatteries 0.20 — 4.5 98 0.6 H batteries 0.05 0.03 4.5 100 0.6 Ibatteries 0.01 0.01 2.0 100 1.0 J batteries 0.50 0.20 7.0 98 0.2 K^((note 1))indexes when the discharge time of batteries “A” at 500 mA(an end-of-discharge voltage of 2.0 V) is 100 ^((note 2)) valuesmeasured at a sinusoidal alternating current of 1 kHz by supplying acurrent of 0.1 mA

According to Table 1, the batteries “F” have a low initial dischargeperformance. The reason for this is considered that the high sodiumcontent of the electrolytic manganese dioxide used as the positiveelectrode active material causes the sodium contained in positiveelectrode 1 to be deposited on negative electrode 2 and to form aresistance film thereon, thereby impeding the discharge reaction.

Table 1 also shows that the batteries “G”, “H”, “I”, and “J” haveexcellent initial discharge characteristics, but have high internalresistances when continued to be discharged at 300 kΩ for one year,indicating that their long-term discharge performance are poor. Thereason for this is considered as follows. Since the electrolyticmanganese dioxides used as the positive electrode active material havepHs of less than 5, the acid generated from the reaction betweensulfuric acid components remaining in the electrolytic manganese dioxideand a small amount of water in the batteries causes manganese ions toelute from positive electrode 1, and to be deposited on negativeelectrode 2, thus forming a resistance film.

In contrast, the batteries “A” to “E” and “K” have low internalresistances when continued to be discharged at 300 kΩ for one year,indicating their excellent long-term discharge performance as well asexcellent initial discharge performance. The batteries “K”, however,require more water than batteries “A” to “E” because the washing isperformed using 40 kg of water per 1 kg of the heat-treated electrolyticmanganese dioxide. In conclusion, in order to save water for thewashing, the sodium content of the neutralized electrolytic manganesedioxide is preferably 0.1 to 0.4% by mass.

The above description shows cylindrical batteries having a spiral-woundelectrode assembly, but the present invention is not limited to such anelectrode assembly or battery shape. The present invention is alsoapplicable to batteries with stacked electrodes or prismatic or coinshaped batteries.

INDUSTRIAL APPLICABILITY

The sodium-neutralized electrolytic manganese dioxide according to thepresent invention can be a positive electrode active material forlithium primary batteries excellent in both initial dischargecharacteristics and long-term discharge characteristics, thus beingsuitable for electrical devices which are required to have a long life.

1. Electrolytic manganese dioxide for lithium primary batteries, theelectrolytic manganese dioxide having a sodium content of 0.05 to 0.25%by mass, and a pH of 5 to 7 as measured according to JIS-K-1467.
 2. Theelectrolytic manganese dioxide for lithium primary batteries of claim 1,wherein the electrolytic manganese dioxide has a sodium content of 0.05to 0.2% by mass.
 3. A lithium primary battery comprising: a positiveelectrode including the electrolytic manganese dioxide for lithiumprimary batteries of claim 1; a negative electrode including one oflithium metal and a lithium alloy; and a separator and a non-aqueouselectrolytic solution between the positive electrode and the negativeelectrode.
 4. The lithium primary battery of claim 3, wherein theelectrolytic manganese dioxide has a sodium content of 0.05 to 0.2% bymass.